The present invention relates to the production of a skinless or low skin flexible polyurethane foam slab using a continuous foaming process.
During the continuous production of free rise flexible slabstock polyurethane foam, a dense skin forms on the top surface of the foam slab. The skin varies in thickness depending upon the chemical formulation used to form the foam, the environmental conditions and other processing parameters. The top skin forms on free-rise polyurethane flexible foams formed using various processing equipment, including, inter alia, MAXFOAM(trademark), VARIMAX(trademark), VARIABLE PRESSURE FOAMING (VPF(trademark)), FOAM-ONE(trademark), EDGEMASTER(trademark), conventional direct laydown, and CO2 processing equipment (i.e., Novaflex(trademark), CarDio(trademark), and Beamech CO-2(trademark)).
Depending upon the formulation, the top skin that forms on the polyurethane slab has a high density and a thickness from 0.01 to 0.25 inches. For many end use applications, the skin must be cut away and discarded as waste, thus resulting in loss of chemicals that could have produced useful polyurethane foam. Depending upon the exact thickness and density of the top skin formed, the formulation chemicals present in the skin typically are sufficient to have produced 0.1 to 2.5 inches of additional foam height. This skin thus represents a significant yield loss in the foaming process.
Apparatus and methods have been proposed in attempts to eliminate the top skin, but they have not proved successful for forming flexible polyurethane slabstock foam. One common method is to apply a paper or polyethylene sheet to the top surface of the foam during the early stages of the foam forming chemical reaction (PINTOMAX(trademark)). Using this PINTOMAX(trademark) process, skin formation may be partly avoided because the sheet forms a barrier between the foam and the atmosphere above the foam. However, the sheet also prevents gases generated during the foaming process (such as CO2, methylene chloride, acetone, pentane or other auxiliary blowing agents) from escaping without damaging the foam cell structure and forming splits and voids on the top surface of the slab.
U.S. Pat. No. 3,816,233 discloses a method for forming a polyurethane sheet with different properties on its top and bottom surfaces produced by differentially heating (or cooling) those surfaces during foam expansion. The patent seeks a foam with a xe2x80x9cporous skinxe2x80x9d. In the preferred embodiment, a top cover sheet placed on the top surface of the foaming mixture is heated with infrared radiation from lamps to temperature from 25xc2x0 to 100xc2x0 F. above the temperature of the bottom surface of the foaming mixture. The preferred heating temperature is 175xc2x0 F. to 200xc2x0 F. for the heated side and 80xc2x0 F. to 120xc2x0 F. for the unheated (or cooled) side. The foam is then compressed to a desired thickness and the cover sheet is removed. The patent thus seeks to form a region of higher density, like a skin on the foam sheet surface, and uses top heating to accomplish this result.
U.S. Pat. No. 3,345,439 describes forming rigid thermoplastic structures. The patent explains that maintaining the temperature of the walls of the structure in which the foam is produced affects the thickness of the skin that is formed on the surface of the material. Lower temperatures produce thicker unexpanded skins. According to the patent, the rising foam is passed between heating platens, and in order to achieve a more gradual expansion, one of the heating platens may be angled such that the heating field has a greater intensity at the leading edge than at the trailing edge. The patent relates only to rigid foams.
Published patent application WO 00/47384 discloses a method and apparatus for producing skinless flexible polyurethane slabstock foam in continuous or molding processes in which the foaming occurs in a closed space with a gas saturated atmosphere. The preferred gas in the gas saturated atmosphere is carbon dioxide, or a mixture of carbon dioxide with other gases. The application suggests it may also be advisable to heat the surface of the enclosure in which foaming occurs to prevent undesired condensation thereon.
Perhaps similar to WO 00/47384, U.S. Pat. 5,804,113 discloses a method for producing slabstock polyurethane foam under controlled pressure and temperature conditions. Free rise expansion of the foaming ingredients takes place within a hermetically enclosed space. The temperature in the enclosed space is controlled by introducing heated air. The air temperature range is from 10xc2x0 C. to 75xc2x0 C., preferably 20xc2x0 C. to 50xc2x0 C. (Col. 5, lines 28-29). The patent does not discuss foam skin formation or methods for eliminating such skin.
The prior art does not teach or suggest a method and apparatus for preventing or reducing the skin formation on a flexible polyurethane slabstock foam without additional gases or foam-forming ingredients or extraneous films. Nor does the prior art teach or suggest a method and apparatus for preventing or reducing the skin formation on a flexible polyurethane slabstock foam produced on an open conveyor.
A method for reducing or eliminating a surface skin when producing a flexible polyurethane foam slab includes the steps of: (a) feeding a chemically reactive polyurethane foaming mixture into a region in which foaming expansion occurs, and (b) heating the top surface of the polyurethane foaming mixture during foaming expansion and/or after such expansion. Preferably, heat is applied only as such foaming expansion occurs.
The chemically reactive polyurethane foaming mixture defines an internal temperature as it expands. The internal temperature escalates as the foaming mixture is conveyed away from the feeding zone of the dispensing device. Preferably, the heating is conducted so as to prevent a significant temperature gradient from forming between the internal temperature and the temperature measured at the top surface of the polyurethane foaming mixture. Heating the top surface to a temperature within aboutxc2x150xc2x0 F. of the internal temperature of the foaming mixture has produced satisfactory results for most polyurethane foams.
Most preferably, the top surface of the polyurethane foaming mixture is heated to a temperature in the range of about 140xc2x0 F. to 350xc2x0 F. Excellent results have been obtained by heating to temperatures in the range of about 230xc2x0 F. to 260xc2x0 F. The preferred temperature is dependent upon the chemical formulation, the processing parameters and the ambient conditions during the process. Heating may also be applied in an escalating fashion, such that the top surface of the foaming mixture is heated to a greater degree as expansion occurs and as the foaming mixture is conveyed along a conveying path away from the dispensing device.
The heating may be supplied by one or a combination of radiant heat lamps, radiant heating panels, by heated gas, preferably air, or other heating sources introduced into the region in which foaming expansion occurs, and that are determined to be appropriate for the foaming chemical formula, process conditions and space limitations to maximize the benefit of skin density reduction. The heat may be applied from the region immediately adjacent to the dispensing device and to a region up to approximately 200 feet downstream and away from the dispensing device. For VARIMAX(trademark) foaming equipment, for example, the heat may be applied to the surface of the foaming mixture starting at the trough and extending up to 200 feet downstream from the trough. For conventional foaming equipment in which the dispensing device does not include a trough, the heat may be applied to the surface of the foaming mixture within about twenty feet after the foaming mixture is laid onto a fall plate or conveyor and heating may be continued up to 200 feet downstream from the dispensing device.
The invention further comprises an apparatus for producing a skinless or low skin flexible polyurethane foam slab, which may have a trough or a dispensing device into which a chemically reactive polyurethane foaming mixture is introduced, a fall plate onto which the foaming mixture is fed from the trough or dispensing device, and a conveyor for conveying the expanded foam away from the fall plate along a conveying path. The apparatus further includes a heating source for heating a top surface of the chemically reactive polyurethane foaming mixture during and/or after foam expansion.
Most preferably, the heating source is a radiant heating panel or a series of radiant heating panels. The radiant heating panels may be mounted for tilting movement with relation to the conveying path. Preferably, the panels are tilted so that each panel is spaced farther from the top surface of the chemically reactive polyurethane foaming mixture at a first position or zone along the conveying path and closer to the top surface of the polyurethane foaming mixture at a second position or zone along the conveying path. Alternatively, the heating source is a heated gas blanket, wherein preferably the gas is air, including but not limited to, ambient air drawn from the atmosphere and heated by heating coils.